truss dob: how do you keep your mirror clean?

I've owned several truss dobs and some tubed dobs. In all cases the tubed primarys stayed very clean for a long time. The truss dob primaries got very dusty and dirty, no matter how carefully I stored the scope. of course, this is to be expected since the truss dob can't be totally sealed up.

Anyone have a good solution for their truss? How do you cover your mirrors when not in use?I suppose you can remove and store the mirror each time but, this adds more steps to setup and takedown, which I don't like.

I'm looking at a 10-11" dob again and this issue is pushing me towards a solid tube design.

Top and bottom covers for the mirror box with low-density weather seal along the inside edge of the cover. It doesn't have to be air-tight. I just put a threaded insert in each corner of the mirror box. After the truss tubes go on, covers get removed.

I made a cell for my 6" that makes it easy to remove or install the primary by simply removing the top knob. The body of the knobs are wide enough so once removed I have enough clearance for mirror removal. They are made of Delrin.

My most often used scope (by far) is my solid tube planetary that I keep near the back door. The strut Dob is primarily to take to the Everglades, but it's not a big deal to put it together or take down.

For my 15" Dob, based on some discussions in the ATM forum re: fans, cooling, & turbulence, i've just created a "baffle" sitting just above my mirror (it's foam-core, & easily removable ); this directs air at & around the mirror more efficiently.

But it also provides a circular aperture wherein a cover can nestle! Used it for the first time last nite. So now, with the scope set up for a few days in-a-row, covered with a "Telegizmos" cover, the mirror itself, down in the mirror-box portion of the scope, is also fairly well sealed. Pix to follow.

Also, some thanks are due to JimMo, as this solution of mine was also partially inspired by HIS cover. This, as well as others posting to the thread he linked above- Dick, Rick, & Joe.

Okay, finally managed to snap some pix of the new baffle i constructed for my Dob. Part of it's implementation was reversing the cooling fan's direction, drawing cooler air from the topside DOWN, past the mirror, and out the back- rather than blowing hot ground-air UP and right at the mirror! The baffle directs the incoming air to more effectively wash the mirror, hopefully removing some boundary turbulence as it goes. The (possible) proof of the pudding was razor-sharp views of Saturn this last weekend (8/10/013)- even with it low in the west with evening breezes blowing! The ring detail was excellent, as was the planets banding, both showing subtle striations of color & light in moments of better seeing. The polar "hood" was easily evident, too, tho i was unable to glimpse its odd shape, even at ~400x.

The photo (below) shows the baffle's "dog-ears" enabling it to pass the pole-mounting hardware... they flop down & flat to seal the perimeter of the box, the foamcore baffle setting upon glued-on balsa wood "ledger" segments at the four box edges. The backside of the baffle is reinforced/stiffen'd using scrap strips of foamcore, also augmenting the control of airflow.

Overall the weight contribution of the components was negligible; we'll see how they hold up in actual use.

Attached Files

To help keep the mirror clean(er?), the baffle's presence enables a drop-in plastic "lid" to seal off the mirror inside the lower portion of the "box". So such a lid, combined with the use of a "Telegizmos 365" cover during the day(s) between viewing sessions, will hopefully relieve the necessity of, or lengthen the period between mirror washings.

The lid is a plastic planter-pot drain basin. So between the 3/4" balsa, the 30"x30" foamcore (ie. "foamboard"), and the "lid", i think the turtle came to ~$20.

Attached Files

Nice job, Mike. I might have to try reversing my fan and see how it works.

To the OP: I decided to stick with a solid tube for my 12 inch for several reasons - cleanliness being farther down on the list.

Unless you are strapped for space or have health issues, I would stick with a solid tube OTA for anything under 12 inches. I'll admit that the truss style looks cool, but I think you might be adding complexity without benefits in a smaller Dob.

At 12 inches, mine is right on the border and there is a good chance that I'll eventually build a truss structure for it.

Anyone have a good solution for their truss? How do you cover your mirrors when not in use?

I have three truss/strut scopes that live in "well ventilated" garage with a bad floor in the high desert where the wind blows most of the time, real western "dust" like you see in the cowboy movies is a fact of life.

I cover my mirrors with a stiff polyethylene, it covers the mirror without touching it (hopefullY) and is tucked in around the sides of the mirror and underneath the mirror cell. I then put a solid cover on top to protect against something falling. I then put a scope cover over the entire rig and make sure it is tucked in around the bottom of the rocker box.

It seems to be effective. In that environment, dust in inevitable but they stay quite clean.

Jon

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For my 14" which has a totally open truss structure down to the mirror, I found a polyethylene toy basket that was just the right size. I cut the bottom out of the basket and it fit snugly over the mirror as seen in the photo. It seems to do an excellent job of keeping the mirror clean.

For my recently rebuilt 20" I tried to find a suitable laundry basket or garbage can but couldn't find anything the right size. So I cut a plywood disk 20" diameter and nailed some pieces of plastic from the rim of that same green toy basket around the edge of the plywood disk. This cover rests on the four mirror safety clips, about 1/2" above the mirror, and the basket rim hugs the edge of the mirror. It seems to be working very well.

The top of my 18" truss dob mirror box has a circular opening, over which a wooden lid fits. This seems to suffice. Loose dust I blow off with an air blower (rubber bulb thing the size of a fist used by photographers)

I tried using an 18" Remo Drumhead to add a second layer of protection, as linked earlier in this thread. Sure, this can be lowered much closer to the primary mirror as pictured here: http://www.cloudynig...rum head 3.jpg.

However, unlike the wooden cover for my mirror box which geometrically can NEVER hit my primary, it is possible for the drum skin to touch the mirror eg if dropped when not centred.

Reputed opticians will say, only when a mirror becomes *excessively* dirty does it actually degrade the view. Just worth bearing in mind. As a % of the mirror surface area dust and dirt will occupy a tiny tiny area.

Interesting thread. Like Jon, my box also has a wooden lid that fits a hole above the primary. One thing I've never seen mentioned is insect encroachment. We all know how caustic insect excrement and spider webs are to the mirror coatings, so that would seem the more important aspect of protection rather than dust. How do people seal their truss dobs tight enough to keep insects from sneaking in and fouling the mirror surface? Does anyone use some sort of elastic cover on the bottom end of the box to seal around the cell?

Twenty five years ago a friend uncovered his 25" that was stored in a shed, and found that a family of mice was living on his mirror surface and had made quite a mess. That really made me appreciate how I have always built my scopes so the mirror could be totally closed. I built a removable dust cover that sits an inch above the mirror. The back of the scope has large cooling holes that can be covered when a piece of plastic is rotated to cover the holes. No open back metal cells for me. If you must - make a dust cover that will cover it up when the scope is not in use.

Also, a simple dust cover is made to fit over the diagonal, not a bag that can touch the coatings. Even my 42" observatory scope is built the same way. that mirror only has to be cleaned every couple of years.